The present invention relates to a laser beam welding process using a gas mixture consisting of nitrogen and helium in proportions that are adjusted or adapted according to the power or power density of the laser device used.
In industry, it is known to use a laser beam to cut or weld one or more metal workpieces. In this regard, the following documents may be cited: DE-A-2 713 904, DE-A-4 034 745, JP-A-01048692, JP-A-56122690, WO 97/34730, JP-A-01005692, DE-A-4 123 716, JP-A-02030389, U.S. Pat. No. 4,871,897, JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693, JP-A-15694, JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553, WO-A-98/14302, DE-A-3 619 513 and DE-A-3 934 920.
Laser welding is a very high-performance welding process as it makes it possible to obtain, at high speeds, very great penetration depths compared with other more conventional processes, such as plasma welding, MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding.
This is explained by the high power densities involved when focusing the laser beam by one or more mirrors or lenses in the joint plane of the workpieces to be welded, for example power densities that may exceed 106 W/cm2.
These high power densities cause considerable vaporization at the surface of the workpieces which, expanding to the outside, induces progressive cratering of the weld pool and results in the formation of a narrow and deep vapor capillary called a “keyhole” in the thickness of the plates, that is to say in the joint plane.
This capillary allows the energy of the laser beam to be directly deposited depthwise in the plate, as opposed to the more conventional welding processes in which the energy deposition is localized on the surface.
This capillary is formed from a metal vapor/metal vapor plasma mixture, the particular feature of which is that it absorbs the laser beam and therefore traps the energy within the actual capillary.
One of the problems with laser welding is the formation of a shielding gas plasma.
This is because this metal vapor plasma, by seeding the shielding gas with free electrons, may induce the appearance of a shielding gas plasma, which is prejudicial to the welding operation.
The incident laser beam may therefore be greatly, or even totally, absorbed and therefore may lead to a substantial reduction in the penetration depth, or even in a loss of coupling between the beam and the material and therefore a momentary interruption in the welding process.
The power density threshold at which the plasma appears depends on the ionization potential of the shielding gas used and is inversely proportional to the square of the wavelength of the laser beam.
Thus, it is very difficult to weld under pure argon with a CO2-type laser, whereas this operation may be carried out with very much less of a problem with a YAG-type laser.
In general, in CO2 laser welding, helium is used as shielding gas, this being a gas with a high ionization potential and making it possible to prevent the appearance of the shielding gas plasma, and to do so irrespective of the laser beam power employed.
However, helium has the drawback of being an expensive gas and many laser users prefer to use other gases or gas mixtures that are less expensive than helium but which would nevertheless limit the appearance of the shielding gas plasma and therefore obtain welding results similar to those obtained with helium, but at a lower cost.
Thus, gas mixtures are commercially available that contain argon and helium, for example the gas mixture containing 30% helium by volume and the rest being argon, sold under the name LASAL™ 2045 by L'Air Liquide™, which make it possible to achieve substantially the same results as helium, for CO2 laser power levels below 5 kW and provided that the power densities generated are not too high, that is to say above about 2000 kW/cm2.
However, the problem that arises with this type of Ar/He mixture is that it is no longer suitable for higher laser power densities, since the threshold at which the shielding gas plasma is created is then exceeded.
It is an object of the present invention therefore to solve this problem by proposing an improved laser welding process that can employ lasers with a power exceeding 15 to 20 kW and to do so, with no or minimal shielding gas plasma formation, irrespective of the power or power density chosen.